1. Field of the Invention
This invention relates to a method of digital signal transmission and, more particularly, to such method suitable for serial transmission of PCM digital audio signals of a large number of channels, such as 8 to 32 channels.
2. Description of Related Art
In recent years, it is becoming an accepted practice to convert analog signals, such as audio or voice signals, into digital signals by, for example, pulse code modulation system (PCM system) and to transmit the resulting digital signals through a signal transmission medium or to record or reproduce them on or from a recording medium. In the digital audio signals, one word is a sequence of bits of digital audio sample data obtained by quantizing and encoding a sample of the original analog signal. In addition to these digital audio sample data, it may be occasionally necessary to transmit error protection data, user definable alphanumeric data, channel status data inclusive of various information as later described, or an information used to check for reliability of the sample data. In addition, it may become occasionally desirable to be able to transmit other data useful in future application or data inferior in quality but useful for auxiliary digital audio channels.
The method of serially transmitting these digital signals containing these various data is required to satisfy the following conditions.
First of all, the transmission waveform need be DC-free in order that transmission may take place with or without the intermediary of AC coupling, and also that the threshold values may be positively detected in the case of an electrical system. In addition, the transmission waveform need also be polarity-free in order that transmission signals may remain unaffected even when the cables of, for example, a twisted pair are interchanged one with the other, the apparatus being additionally simplified in structure. The mechanical aspect should also be easy and simple and, in case of application to a broadcasting station or studio, should be adaptable to transmission distance requirements.
According to this technology, the PCM digital audio signals of a few channels, such as 1 to 4 channels, may be transmitted serially with high reliability and efficiency.
When serially transmitting digital signals of a large number of, e.g. 8 to 32 channels, through an optical fiber or other high-speed transmission medium, in view of the increased bit rate, it is desirable to make use of digital modulation systems which reduce the bandwidth required of the transmission medium. Since optical signal transmission from one point to another takes place at an approximately constant power level, the merits or demerits of using DC-free or polarity-free waveforms are cancelled. In addition, in fast serial transmission of multichannel digital data, some of the aforementioned requirements may preferably or should be modified.
In view of the foregoing, it is a principal object of the present invention to provide a method suited for serial transmission of PCM digital audio signals of a large number of channels, such as 8 to 32 channels, on a high-speed transmission medium, such as optical fiber.